These appear to be unused for extension fields, so we're free to change the mapping without breaking anything.
As the TODO says, the mapping that's currently implemented doesn't seem natural or useful. It seems more natural to treat the `BigUint` as a base field element, potentially in a non-canonical form.
* Split into crates
I kept other changes to a minimum, so 95% of this is just moving things. One complication that came up is that since `PrimeField` is now outside the plonky2 crate, these two impls now conflict:
```
impl<F: PrimeField> From<HashOut<F>> for Vec<u8> { ... }
impl<F: PrimeField> From<HashOut<F>> for Vec<F> { ... }
```
with this note:
```
note: upstream crates may add a new impl of trait `plonky2_field::field_types::PrimeField` for type `u8` in future versions
```
I worked around this by adding a `GenericHashOut` trait with methods like `to_bytes()` instead of overloading `From`/`Into`. Personally I prefer the explicitness anyway.
* Move out permutation network stuff also
* Fix imports
* Fix import
* Also move out insertion
* Comment
* fmt
* PR feedback
